Watching Steven Spielberg's films often demands a leap of faith, and War Horse requires more leaps than Becher's Brook.

Among us art-house types, it's long been the default position to regard Spielberg as the Great Satan, or at least as a corporate entity who spends most of his energy executively rubber-stamping Hollywood's high-profile money-spinners. But occasionally Spielberg reminds us that he's still a hugely individual director with a penchant for risk-taking.

War Horse is not one of Spielberg's "experimental" films: it's not A.I. or Munich or his surprisingly dark War of the Worlds. It's what you'd call a family film – not in the usual Hollywood sense of a kids' entertainment with added frills to amuse grown-ups, but an essentially adult one that requires you to accept a childlike perspective for it really to work.

In Michael Morpurgo's children's novel War Horse, the horse is the narrator. I don't know how it works in the acclaimed stage adaptation, but Spielberg and writers Lee Hall and Richard Curtis – a team that sounds at first like really bad news – take a straightforward tack. They simply follow Joey the (non-speaking) horse from foalhood on a Devon farm to his ordeals on the battlefields of the First World War – a journey from pastoral heaven to earthly hell.

For the first 15 minutes, I was sceptical. The rolling landscapes are impossibly lovely, in an England populated by hoarily whiskered rurals – Peter Mullan, David Thewlis et al, with Emily Watson as long-suffering, warm-hearted Ma, fists on aproned hips as she tuts at the menfolk's folly. Spielberg could hardly have chosen a more US-friendly British face to play plucky farmboy Albert than Jeremy Irvine, who – ruddy cheeks notwithstanding – would look perfectly at home in High School Musical 4.

Horse bonds with boy, and the pair triumph in ploughing an unploughable field – and against the odds, War Horse started to win me over. What seems indigestibly Spielbergian – brashly optimistic and sentimental – emerges with increasing confidence as a knowingly retro exercise. With its sun-steeped horizons and excessively lush score (John Williams does Vaughan Williams), War Horse adopts the bygone language of Hollywood high style – John Ford, Gone with the Wind, National Velvet – and dares you to resist its emotional directness.

The film continues to offer hostages to cynicism. When the British officer who buys Joey promises Albert "man to man" to take care of him, there were a few snorts and whinnies in the screening room – but only Tom Hiddleston could have the blue-eyed candour to pull this line off. Then the film heads into darker areas. In an extraordinary bravura sequence, Benedict Cumberbatch's cavalry division launches an assault on German lines, an episode that ends with a sobering vista of thousands of dead men and horses.

Despite occasional close-ups of Joey that we're meant to read as sorrow, compassion or comradeship for another horse, Spielberg doesn't make heavy weather of the anthropomorphism: even he knows a horse is a horse. What's remarkable is that supposed protagonist Albert drops out early on and the film drifts with Joey from human to human, most of them doomed – like two German privates who briefly capture our sympathy before being shot as deserters.

Not all these episodes work – there's a horribly twee interlude on a French farm that's pure Stella Artois ad. Still, this succession of diverse faces flouts prevailing Hollywood logic, which prefers that we stick to one likeable character throughout a film.

The Somme sequences are extraordinary, especially the genuinely infernal episode in which Joey bolts into No Man's Land, becoming entangled in barbed wire. This is nothing less than an equine Passion of the Christ, and a Christ-like resurrection follows, as a British and a German soldier collaborate to cut the horse free.

Joey's entire story is a string of unlikely reprieves; he's forever facing the bullet, the knacker's yard or worse, and the very idea that it should matter that a single horse survives the Somme – well, it's palpably as ludicrous as that of rescuing one soldier in Saving Private Ryan. You could see Joey's survival story as a pious reassuring lie about war – although War Horse hardly pulls its punches on the slaughter – or you can take it as a fable about the redeeming power of compassion.

You just have to be prepared to buy into Spielberg's wholehearted espousal of an archaic film language – given, of course, a modern widescreen reboot. Through sheer cumulative effect, and some superb acting, War Horse transcends manipulativeness and sentiment to become a very powerful film indeed. War Horse is flawed for sure, but it contains more brio, more surprises, and – dare I say it – more sheer cinema than you might expect.